forward hundreds of mail-addresses without having user/pop3 account

Hi all, what would be the best way to forward hundreds mailadressess like

user_1...500@mydomain.tld ​

to different mail targets on external systems like

postbox_1...500@externaldomain_a-z.tld ​

by using Maildir and without creating hundreds of user-accounts for all the incoming addresses on my system? (the target addresses are all on different target systems!)
(see showthread.php?t=1867)

I checked different possibilities like /etc/aliases, /etc/transport but I´m not sure what´s the best way or if there are any hidden traps. And at this time it is not clear for me if a full qualificated email-address can be used as target in these postfix files.

man transport uses hops/hosts/etc. as targets, no fq. mail addresses, so I´m not sure if I can use that. Even in google-search I did not found clearly syntax examples (for exactly this case)